MOBILITY AND PLANT UPTAKE OF INORGANIC C-14 AND C-14-LABELED PCB IN SOILS OF HIGH AND LOW RETENTION

Quantifying and understanding the mobility of C-14 and organic pollutants in soils is important, especially in the context of underground waste disposal. We studied migration of C-14 applied as NaHCO3 (C-14-CO3) and as 2,2',5,5' tetrachlorobiphenyl (C-14-PCB) in carbonated, high-organic-matter-content and acidic, low-organic-matter-content undisturbed soil cores. The mobility of C-14-PCB depends on the profile distribution and amount of soil organic matter, whereas the mobility of C-14-CO3 depends primarily on the soil carbonate content. The solid/liquid partition coefficients (K(d)) for C-14-CO3 were 6.7 and 1.2 mL g-1 for the two soils, respectively. For the C-14-PCB, the corresponding K(d) values were 49 and 22 mL g-1. Plant/soil concentration ratios (CR) for inorganic C-14 have previously been derived using overly conservative assumptions. Using plants grown in outdoor lysimeters, CR values for C-14-CO3 of 0.7 and 1.3, on a dry-weight basis, were measured for the two soils. These values are about 25-fold lower than the currently used values. The corresponding CR values for C-14-PCB were 0.014 and 0.088. For both C-14 sources, there was evidence of atmospheric transfer from the soil to the plants. This was especially important for C-14-CO3, where it may have been dominant. Detailed modelling of C-14 transport from underground waste disposal should include volatilization as a loss process from soil as well as a source for plants.